Method of regenerating ruthenium catalysts for the hydrogenation of benzene

ABSTRACT

The present patent application describes a method of regenerating a ruthenium catalyst for the hydrogenation of benzene, which comprises flushing the catalyst with inert gas in a regeneration step until the original activity or part of the original activity has been attained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of PCT/EP2007/057758 filed Jul. 27,2007 which in turn claims priority from European Application 06118202.8filed Jul. 31, 2006, the entire contents of which are incorporatedherein by reference.

The present invention relates to a method of regenerating a catalystwhich is used for the hydrogenation of benzene to cyclohexane.

A particularly useful catalyst which can be used in the hydrogenation ofaromatic compounds is disclosed in DE 196 24 485 A1. The catalystcomprises, as active metal, either ruthenium alone or ruthenium togetherwith at least one metal of transition group I, VII or VIII of thePeriodic Table (CAS version) in an amount of from 0.01 to 30% by weight,based on the total weight of the catalyst, applied to a support. From 10to 50% of the pore volume of the support is formed by macropores havinga pore diameter in the range from 50 nm to 10 000 nm and from 50 to 90%of the pore volume of the support is formed by mesopores having a porediameter in the range from 2 to 50 nm, with the sum of the pore volumesbeing 100%. Supports used are activated carbon, silicon carbide,aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide,magnesium dioxide, zinc oxide or a mixture of two or more thereof.

Further particularly useful catalysts for the hydrogenation of aromaticcompounds are disclosed in EP-A 1 169 285. In one embodiment (catalyst1), the catalyst comprises at least one metal of transition group VIIIof the Periodic Table applied to a support, with the support havingmacropores and the catalyst comprises, as active metal, at least onemetal of transition group VIII of the Periodic Table, either alone ortogether with at least one metal of transition group I or VII of thePeriodic Table (CAS version), applied to a support, with the supporthaving a mean pore diameter of at least 50 nm and a BET surface area ofnot more than 30 m²/g and the amount of active metal being from 0.01 to30% by weight, based on the total weight of the catalyst. In a furtherembodiment (catalyst 2), the catalyst comprises, as active metal, atleast one metal of transition group VIII of the Periodic Table, eitheralone or together with at least one metal of transition group I or VIIof the Periodic Table in an amount of from 0.01 to 30% by weight, basedon the total weight of the catalyst, applied to a support, with from 10to 50% of the pore volume of the support being formed by macroporeshaving a pore diameter in the range from 50 nm to 10 000 nm and from 50to 90% of the pore volume of the support being formed by mesoporeshaving a pore diameter in the range from 50 nm to 10 000 nm and from 50to 90% of the pore volume of the support being formed by mesoporeshaving a pore diameter in the range from 2 to 50 nm, with the sum of theproportions of the pore volumes being 100%. Supports used are activatedcarbon, silicon carbide, aluminum oxide, silicon dioxide, titaniumdioxide, zirconium dioxide, magnesium dioxide, zinc oxide or a mixtureof two or more thereof, preferably aluminum oxide.

Finally, a further particularly useful catalyst is disclosed in thepatent application DE 102 005 029 200. This is a coated catalystcomprising, as active metal, either ruthenium alone or rutheniumtogether with at least one further metal of transition group IB, VIIB orVIII of the Periodic Table of the Elements (CAS version) applied to asupport comprising silicon dioxide as support material, wherein theamount of active metal is <1% by weight, based on the total weight ofthe catalyst, and at least 60% by weight of the active metal is presentin the shell of the catalyst to a penetration depth of 200 μm,determined by means of SEM-EPMA (EXDS).

The maintenance of the catalyst activity over a very long period of timeis of great economic importance for industrial processes.

A decrease in the catalytic activity is usually caused by variousphysical and chemical effects on the catalyst, for example by blockingof the catalytically active sites or by loss of catalytically activesites as a result of thermal, mechanical or chemical processes. Forexample, catalyst deactivation or aging in general can be caused bysintering of the catalytically active sites, by loss of (noble) metal,as a result of deposits or by poisoning of the active sites. There aremany aging/deactivation mechanisms.

Conventionally, the deactivated catalyst has to be removed from thereactor for regeneration. The reactor is then down, or operation isresumed after installation of another catalyst or switching over to apreviously installed further catalyst. In either case, this leads tosignificant costs. The U.S. Pat. No. 3,851,004 and U.S. Pat. No.2,757,128 disclose processes for the hydrogenation of, inter alia,olefins in hydrocarbon starting materials and the regeneration of thecatalysts by means of hydrogen.

DE 196 34 880 C2 discloses a process for the simultaneous selectivehydrogenation of diolefins and nitriles from a hydrocarbon startingmaterial. In this process, the catalyst is, after its diolefinhydrogenation activity has dropped to less than 50% of the initialactivity, flushed with an inert gas to remove traces of the hydrocarbonfrom the catalyst and to produce a flushed catalyst and this is flushedwith hydrogen in a subsequent regeneration step. This produces aregenerated catalyst whose diolefin hydrogenation activity is once againat least 80% of the initial value.

Deactivation is likewise observed in the hydrogenation of benzene usingthe ruthenium catalysts described, and this deactivation has not yetbeen able to be overcome in a simple way.

It is an object of the present invention to provide a method ofregenerating a ruthenium catalyst used in the hydrogenation of benzene.This should be simple to implement in terms of apparatus and beinexpensive to carry out. In particular, multiple and completeregeneration of the catalyst is sure to be able to be achieved thereby.

The above object is achieved by a method of regenerating a rutheniumcatalyst for the hydrogenation of benzene, which comprises flushing thecatalyst with inert gas in a regeneration step until the originalactivity or part of the original activity has been attained.

This regeneration firstly results in higher conversions due to anincreased catalyst activity, and, secondly, the catalyst operating livesin production operation are significantly increased by means of themethod of the invention.

The method of the invention is particularly suitable for regeneration ofRu catalysts which are described in the patent applications EP-A 0 814098, EP-A 1 169 285 and DE 102 005 029 200 and are used in the processesdisclosed there. These catalysts and processes are described below.

In all of the present patent application, the groups of the PeriodicTable are designated according to the CAS version.

Preferred Catalysts

EP-A 0 814 098

The catalysts described below are designated as “catalyst variant I” inthe present patent application.

As active metals, it is in principle possible to use all metals oftransition group VIII of the Periodic Table. Preference is given tousing platinum, rhodium, palladium, cobalt, nickel or ruthenium or amixture of two or more thereof as active metals, with particularpreference being given to using ruthenium as active metal.

The terms “macropores” and “mesopores” are, for the purposes of thepresent invention, used in accordance with the definition in Pure Appl.Chem., 45, p. 79 (1976), namely pores whose diameter is above 50 nm(macropores) or whose diameter is in the range from 2 nm to 50 nm(mesopores). “Micropores” are likewise defined in the references citedabove and denote pores having a diameter of <2 nm.

The active metal content is generally from about 0.01 to about 30% byweight, preferably from about 0.01 to about 5% by weight and inparticular from about 0.1 to about 5% by weight, in each case based onthe total weight of the catalyst used.

The total metal surface area in catalyst variant I is preferably fromabout 0.01 to about 10 m²/g, more preferably from about 0.05 to about 5m²/g and in particular from about 0.05 to about 3 m²/g, of the catalyst.The metal surface area is determined by means of the chemisorptionmethod described by J. Lemaitre et al. in “Characterization ofHeterogeneous Catalysts”, editor. Francis Delanney, Marcel Dekker, NewYork 1984, pp. 310-324.

In catalyst variant I, the ratio of the surface areas of the activemetal/metals and the catalyst support is preferably less than about0.05, with the lower limit being about 0.0005.

Catalyst variant I comprises a support material which is macroporous andhas a mean pore diameter of at least about 50 nm, preferably at leastabout 100 nm, in particular at least about 500 nm, and whose BET surfacearea is not more than about 30 m²/g, preferably not more than about 15m²/g, more preferably not more than about 10 m²/g, in particular notmore than about 5 m²/g and more preferably not more than about 3 m²/g.The mean pore diameter of the support is preferably from about 100 nm toabout 200 μm, more preferably from about 500 nm to about 50 μm. The BETsurface area of the support is preferably from about 0.2 to about 15m²/g, more preferably from about 0.5 to about 10 m²/g, in particularfrom about 0.5 to about 5 m²/g and more preferably from about 0.5 toabout 3 m²/g.

The surface area of the support is determined by the BET method by meansof N₂ adsorption, in particular in accordance with DIN 66131. Thedetermination of the mean pore diameter and the pore size distributionis carried out by means of Hg porosimetry, in particular in accordancewith DIN 66133.

The pore size distribution of the support can preferably beapproximately bimodal, with the pore diameter distribution having maximaat about 600 nm and about 20 μm in the bimodal distribution representinga specific embodiment of the invention.

Further preference is given to a support which has a surface area of1.75 m²/g and has this bimodal distribution of the pore diameter. Thepore volume of this preferred support is preferably about 0.53 ml/g.

As macroporous support material, it is possible to use, for example,macropore-comprising activated carbon, silicon carbide, aluminum oxide,silicon dioxide, titanium dioxide, zirconium dioxide, magnesium oxide,zinc oxide or mixtures of two or more thereof, with particularpreference being given to using aluminum oxide and zirconium dioxide.

Corresponding catalyst supports and methods of producing them aredisclosed in the following documents:

Fundamentals of Industrial Catalytic Processes, R. J. Farrauto, C. H.Bartholomew, First Edition 1997, pages 16, 17, 57 to 62, 88 to 91, 110to 111; Oberlander, R. K., 1984 Aluminas for Catalysts, in AppliedIndustrial Catalysis, e.g. D. E. Leach, Academic Press, Vol. 3, Chapter4; U.S. Pat. No. 3,245,919; WO 93/04774; EP-A 0 243 894; Ullmann'sEncyclopedia of Industrial Chemistry, 5th Ed., Vol. AI, p. 588 to 590;VCH 1985.

EP-A 1 169 285

The catalysts described below are designated as “catalyst variant II” inthe present patent application. There are various subvariants of thisvariant II.

Subvariant 1

This catalyst corresponds to that described above under EP-A 0 814 089.

A description is also given of the subvariant 1a used according to theinvention, which represents a preferred embodiment of subvariant 1.Support materials which can be used are ones which are macroporous andhave a mean pore diameter of at least 0.1 μm, preferably at least 0.5μm, and a surface area of not more than 15 m²/g, preferably not morethan 10 m²/g, particularly preferably not more than 5 m²/g, inparticular not more than 3 m²/g. The mean pore diameter of the supportused there is preferably in the range from 0.1 to 200 μm, in particularfrom 0.5 to 50 μm. The surface area of the support is preferably from0.2 to 15 m²/g, particularly preferably from 0.5 to 10 m²/g, inparticular from 0.5 to 5 m²/g, especially from 0.5 to 3 m²/g, of thesupport. This catalyst, too, has the above-described bimodality of thepore diameter distribution with the analogous distributions and thecorrespondingly preferred pore volume. Further details regardingsubvariant 1a may be found in DE-A 196 04 791.9 whose contents are fullyincorporated by reference into the present patent application.

Subvariant 2

Subvariant 2 comprises one or more metals of transition group VIII ofthe Periodic Table as active component(s) on a support as definedherein. Ruthenium is preferably used as active component.

The total metal surface area on the catalyst is preferably from 0.01 to10 m²/g, particularly preferably from 0.05 to 5 m²/g and more preferablyfrom 0.05 to 3 m²/g, of the catalyst. The metal surface area wasmeasured by the chemisorption method described in J. Lemaitre et al.,“Characterization of Heterogeneous Catalysts”, Editor: Francis Delanney,Marcel Dekker, New York (1984), pp. 310-324.

In subvariant 2, the ratio of the surface areas of the at least oneactive metal and the catalyst support is less than about 0.3, preferablyless than about 0.1 and in particular about 0.05 or less, with the lowerlimit being about 0.0005.

The support materials which can be used in subvariant 2 have macroporesand mesopores.

The supports which can be used have a pore distribution in which fromabout 5 to about 50%, preferably from about 10 to about 45%, morepreferably from about 10 to about 30% and in particular from about 15 toabout 25%, of the pore volume is formed by macropores having porediameters in the range from about 50 nm to about 10 000 nm and fromabout 50 to about 95%, preferably from about 55 to about 90%, morepreferably from about 70 to about 90% and in particular from about 75 toabout 85%, of the pore volume is formed by mesopores having a porediameter of from about 2 to about 50 nm, with the sum of the proportionsof the pore volumes in each case being 100%.

The total pore volume of the supports used is from about 0.05 to 1.5cm³/g, preferably from 0.1 to 1.2 cm³/g and in particular from about 0.3to 1.0 cm³/g. The mean pore diameter of the supports used according tothe invention is from about 5 to 20 nm, preferably from about 8 to about15 nm and in particular from about 9 to about 12 nm.

The surface area of the support is preferably from about 50 to about 500m²/g, more preferably from about 200 to about 350 m²/g and in particularfrom about 250 to about 300 m²/g, of the support.

The surface area of the support is determined by the BET method by meansof N₂ adsorption, in particular in accordance with DIN 66131. Thedetermination of the mean pore diameter and the size distribution iscarried out by means of Hg porosimetry, in particular in accordance withDIN 66133.

Although it is in principle possible to use all support materials knownin catalyst production, i.e. those which have the above-defined poresize distribution, preference is given to using activated carbon,silicon carbide, aluminum oxide, silicon dioxide, titanium dioxide,zirconium dioxide, magnesium oxide, zinc oxide and mixtures thereof,more preferably aluminum oxide and zirconium dioxide.

DE 102 005 029 200

The catalysts disclosed below are designated as catalyst variant III or“coated catalysts” in the present patent application.

The subject matter is a coated catalyst comprising, as active metal,either ruthenium alone or ruthenium together with at least one furthermetal of transition group IB, VIIB or VIII of the Periodic Table of theElements (CAS version) applied to a support comprising silicon dioxideas support material.

In this coated catalyst, the amount of active metal is <1% by weight,preferably from 0.1 to 0.5% by weight, particularly preferably from 0.25to 0.35% by weight, based on the total weight of the catalyst, and atleast 60% by weight, particularly preferably 80% by weight, of theactive metal, based on the total amount of active metal, is present inthe shell of the catalyst to a penetration depth of 200 μm. The datagiven above are determined by means of SEM (scanning electronmicroscopy) EPMA (electron probe microanalysis)-EDXS (energy dispersiveX-ray spectroscopy) and represent mean values. Further informationregarding the above-described measurement methods and techniques aredisclosed, for example, in “Spectroscopy in Catalysis” by J. W.Niemantsverdriet, VCH, 1995.

In the coated catalyst, the predominant amount of the active metal ispresent in the shell to a penetration depth of 200 μm, i.e. near thesurface of the coated catalyst. In contrast, no active metal or only avery small amount of active metal is present in the interior (core) ofthe catalyst. It has surprisingly been found that the catalyst variantIII has, despite the small amount of active metal, a very high activityin the hydrogenation of organic compounds comprising hydrogenatablegroups, in particular in the hydrogenation of carbocyclic aromaticgroups, at very good selectivities. In particular, the activity ofcatalyst variant III does not decrease over a long hydrogenation time.

Very particular preference is given to a coated catalyst in which noactive metal can be detected in the interior of the catalyst, i.e.active metal is present only in the outer shell, for example in a zoneto a penetration depth of from 100 to 200 μm.

In a further particularly preferred embodiment, active metal particlescan be detected only in the outermost 200 μm, preferably 100 μm, veryparticularly preferably 50 μm (penetration depth), of the coatedcatalysts by means of (FEG)-TEM (field emission gun-transmissionelectron microscopy) with EDXS.

As active metal, it is possible to use either ruthenium alone orruthenium together with at least one further metal of transition groupIB, VIIB or VIII of the Periodic Table of the Elements (CAS version).Further active metals which are suitable in addition to ruthenium are,for example, platinum, rhodium, palladium, iridium, cobalt or nickel ora mixture of two or more thereof. Among the metals of transition groupsIB and/or VIIB of the Periodic Table of the Elements which can likewisebe used, suitable metals are, for example, copper and/or rhenium.Preference is given to using ruthenium alone as active metal or togetherwith platinum or iridium in the coated catalyst; very particularpreference is given to using ruthenium alone as active metal.

The coated catalyst displays the abovementioned very high activity at alow loading with active metal of <1% by weight, based on the totalweight of the catalyst. The amount of active metal in the coatedcatalyst according to the invention is preferably from 0.1 to 0.5% byweight, particularly preferably from 0.25 to 0.35% by weight. It hasbeen found that the penetration depth of the active metal into thesupport material is dependent on the loading of the catalyst variant IIIwith active metal. Even at a loading of the catalyst variant III with 1%by weight or more, e.g. at a loading with 1.5% by weight, a significantamount of active metal is present in the interior of the catalyst, i.e.at a penetration depth of from 300 to 1000 μm, and this impairs theactivity of the hydrogenation catalyst, in particular the activity overa long hydrogenation time, particularly in the case of fast reactions,with a deficiency of hydrogen being able to occur in the interior of thecatalyst (core).

In the coated catalyst, at least 60% by weight of the active metal,based on the total amount of active metal, is present in the shell ofthe catalyst to a penetration depth of 200 μm. Preference is given to atleast 80% by weight of the active metal in the coated catalyst, based onthe total amount of active metal, being present in the shell of thecatalyst to a penetration depth of 200 μm. Very particular preference isgiven to a coated catalyst in which no active metal can be detected inthe interior of the catalyst, i.e. active metal is present only in theoutermost shell, for example in a zone to a penetration depth of from100 to 200 μm. In a further preferred embodiment, 60% by weight,preferably 80% by weight, based on the total amount of active metal, ispresent in the shell of the catalyst to a penetration depth of 150 μm.The abovementioned data are determined by means of SEM (scanningelectron microscopy) EPMA (electron probe microanalysis)-EDXS (energydispersive X-ray spectroscopy) and are mean values. To determine thepenetration depth of the active metal particles, a number of catalystparticles (e.g. 3, 4 or 5) are cut and ground perpendicular to theextrudate axis (when the catalyst is in the form of extrudates). Theprofiles of the active metal/Si concentration ratios are then determinedby means of line scans. On each measurement line, a number, for examplefrom 15 to 20, measurement points at equal intervals are measured; thesize of the measurement spot is about 10 μm*10 μm. After integration ofthe amount of active metal over the depth, the frequency of the activemetal in a zone can be determined.

Very particular preference is given to the amount of active metal, basedon the concentration ratio of active metal to Si, on the surface of thecoated catalyst determined by means of SEM EPMA-EDXS being from 2 to25%, preferably from 4 to 10%, particularly preferably from 4 to 6%. Thesurface analysis is carried out by means of analyses of regions havingdimensions of 800 μm×2000 μm at an information depth of about 2 μm. Theelemental composition is determined in % by weight (normalized to 100%).The mean concentration ratio (active metal/Si) is determined over 10measurement regions.

For the purposes of the present invention, the surface of the coatedcatalyst is the outer shell of the catalyst to a penetration depth ofabout 2 μm. This penetration depth corresponds to the information depthin the abovementioned surface analysis.

Very particular preference is given to a coated catalyst in which theamount of active metal, both on the weight ratio of active metal to Si(weight/weight in %), on the surface of the coated catalyst is from 4 to6%, at a penetration depth of 50 μm is from 1.5 to 3% and in apenetration depth range from 50 to 150 μm is from 0.5 to 2%, determinedby means of SEM EPMA (EDXS). The values specified are mean values.

Furthermore, the size of the active metal particles preferably decreaseswith increasing penetration depth, determined by means of (FEG)-TEManalysis.

The active metal is preferably present either partly or completely incrystalline form in the coated catalyst. In preferred cases, very finelycrystalline active metal can be detected in the shell of the coatedcatalyst by means of SAD (selected area diffraction) or XRD (X-raydiffraction).

The coated catalyst can further comprise alkaline earth metal ions(M²⁺), i.e. M=Be, Mg, Ca, Sr and/or Ba, in particular Mg and/or Ca, veryparticularly preferably Mg. The content of alkaline earth metal ion(s)(M²⁺) in the catalyst is preferably from 0.01 to 1% by weight, inparticular from 0.05 to 0.5% by weight, very particularly preferablyfrom 0.1 to 0.25% by weight, in each case based on the weight of silicondioxide support material.

An important constituent of catalyst variant III is the support materialbased on silicon dioxide, in general amorphous silicon dioxide. In thiscontext, the term “amorphous” means that the proportion of crystallinesilicon dioxide phases is less than 10% by weight of the supportmaterial. However, the support materials used for preparing thecatalysts can have superstructures formed by a regular arrangement ofpores in the support material.

As support materials, it is basically possible to use amorphous types ofsilicon dioxide which comprise at least 90% by weight of silicondioxide, with the remaining 10% by weight, preferably not more than 5%by weight, of the support material also being able to be another oxidicmaterial, e.g. MgO, CaO, TiO₂, ZrO₂, Fe₂O₃ and/or alkali metal oxide.

In a preferred embodiment of the invention, the support material ishalogen-free, in particular chlorine-free, i.e. the halogen content ofthe support material is less than 500 ppm by weight, e.g. in the rangefrom 0 to 400 ppm by weight. Preference is thus given to a coatedcatalyst which comprises less than 0.05% by weight of halide (determinedby ion chromatography), based on the total weight of the catalyst.

Preference is given to support materials which have a specific surfacearea in the range from 30 to 700 m²/g, preferably from 30 to 450 m²/g(BET surface area in accordance with DIN 66131).

Suitable amorphous support materials based on silicon dioxide are knownto those skilled in the art and are commercially available (cf., forexample, O. W. Flörke, “Silica” in Ullmann's Encyclopedia of IndustrialChemistry 6th Edition on CD-ROM). They can be of natural origin or canhave been produced synthetically. Examples of suitable amorphous supportmaterials based on silicon dioxide are silica gels, kieselguhr,pyrogenic silicas and precipitated silicas. In a preferred embodiment ofthe invention, the catalysts have silica gels as support materials.

Depending on the embodiment of the invention, the support material canhave a different form. If the process in which the coated catalysts areused is a suspension process, the support material is usually used inthe form of a fine powder for producing the catalysts. The powderpreferably has particle sizes in the range from 1 to 200 μm, inparticular from 1 to 100 μm. When the coated catalyst according to theinvention is used in fixed beds of catalyst, it is usual to use shapedbodies composed of the support material which can be obtained, forexample, by extrusion, ram extrusion or tableting and can, for example,have the shape of spheres, pellets, cylinders, extrudates, rings orhollow cylinders, stars and the like. The dimensions of the shapedbodies are usually in the range from 0.5 mm to 25 mm. Catalystextrudates having extrudate diameters of from 1.0 to 5 mm and extrudatelengths of from 2 to 25 mm are frequently used. In general, higheractivities can be achieved when using relatively small extrudates, butthese often do not have sufficient mechanical stability in thehydrogenation process. Very particular preference is therefore given tousing extrudates having extrudate diameters in the range from 1.5 to 3mm.

Process for the Hydrogenation of Benzene Using the Catalysts

The above-described catalysts (catalyst variants I, II and III and thesubvariants mentioned) are preferably used as hydrogenation catalyst.They are suitable, in particular, for the hydrogenation of a carbocyclicaromatic group to the corresponding carbocyclic aliphatic group. Here,complete hydrogenation of the aromatic group particularly preferablyoccurs.

According to the invention, this is benzene, with the expressioncomplete hydrogenation referring to a conversion of cyclohexane ofgenerally >98%, preferably >99%, particularly preferably >99.5%, veryparticularly preferably >99.9%, in particular >99.99% and especially>99.995%.

When the above-described catalyst variants I, II and III are used forthe hydrogenation of benzene to cyclohexane, the typical cyclohexanespecifications which require a residual benzene content of <100 ppm(corresponding to a benzene conversion of >99.99%) are thus likewiseadhered to. As indicated, the benzene conversion in a hydrogenation ofbenzene using the coated catalyst according to the invention ispreferably >99.995%.

The present patent application therefore further provides a process forthe hydrogenation of benzene to cyclohexane which comprises aregeneration step in addition to the hydrogenation step.

The hydrogenation process can be carried out in the liquid phase or inthe gas phase. The hydrogenation process of the invention is preferablycarried out in the liquid phase.

The hydrogenation process can be carried out in the absence of a solventor diluent or in the presence of a solvent or diluent, i.e. it is notnecessary to carry out the hydrogenation in solution.

As solvent or diluent, it is possible to use any suitable solvent ordiluent. Possible solvents or diluents are in principle those which areable to dissolve the organic compound to be hydrogenated, preferablycompletely, or mixed completely with this and are inert under thehydrogenation conditions, i.e. are not hydrogenated.

Examples of suitable solvents are cyclic and acyclic ethers, e.g.tetrahydrofuran, dioxane, methyl tert-butyl ether, dimethoxyethane,dimethoxypropane, dimethyldiethylene glycol, aliphatic alcohols such asmethanol, ethanol, n-propanol or isopropanol, n-butanol, 2-butanol,isobutanol or tert-butanol, carboxylic esters such as methyl acetate,ethyl acetate, propyl acetate or butyl acetate, and also aliphatic etheralcohols such as methoxypropanol and cycloaliphatic compounds such ascyclohexane, methylcyclohexane and dimethylcyclohexane.

The amount of solvent or diluent used is not subject to any particularrestrictions and can be selected freely according to requirements, butpreference is given to amounts which lead to a from 3 to 70% strength byweight solution of the organic compound intended for hydrogenation. Theuse of a diluent is advantageous in order to avoid excessive evolutionof heat in the hydrogenation process. Excessive evolution of heat canlead to deactivation of the catalyst and is therefore undesirable.Careful temperature control is therefore advantageous in thehydrogenation process. Suitable hydrogenation temperatures are mentionedbelow.

When a solvent is used, particular preference is given to using, for thepurposes of the invention, the product formed in the hydrogenation,i.e., cyclohexane as solvent, if appropriate together with othersolvents or diluents. In any case, part of the cyclohexane formed in theprocess can be mixed into the benzene still to be hydrogenated.

Based on the weight of the benzene intended for hydrogenation,preference is given to mixing in from 1 to 30 times, particularlypreferably from 5 to 20 times, in particular from 5 to 10 times, theamount of the cyclohexane product as solvent or diluent.

The actual hydrogenation is usually carried out by bringing the organiccompound as liquid phase or gaseous phase, preferably as liquid phase,into contact with the catalyst in the presence of hydrogen. The liquidphase can be passed over a catalyst suspension (suspension process) or afixed bed of catalyst (fixed-bed process).

The hydrogenation can be carried out either continuously or batchwise,with a continuous process being preferred. The process is preferablycarried out in trickle reactors or in the flooded mode of operationaccording to the fixed-bed mode of operation. The hydrogen can be passedover the catalyst either in cocurrent with the solution of the startingmaterial to be hydrogenated or in countercurrent.

Suitable apparatuses for carrying out a hydrogenation over a moving bedor fixed bed of catalyst are known from the prior art, e.g. fromUllmanns Enzyklopädie der Technischen Chemie, 4th edition, volume 13, p.135 ff., and from P. N. Rylander, “Hydrogenation and Dehydrogenation” inUllmann's Encyclopedia of Industrial Chemistry, 5th ed. on CD-ROM.

The hydrogenation can be carried out either under hydrogen atatmospheric pressure or under an increased hydrogen pressure, e.g. at anabsolute hydrogen pressure of at least 1.1 bar, preferably at least 2bar. In general, the absolute hydrogen pressure will not exceed a valueof 325 bar and preferably 300 bar. The absolute hydrogen pressure isparticularly preferably in the range from 1.1 to 300 bar, veryparticularly preferably in the range from 5 to 40 bar. The hydrogenationof benzene is carried out, for example, at a hydrogen pressure ofgenerally=50 bar, preferably from 10 bar to 45 bar, particularlypreferably from 15 to 40 bar.

In the process of the invention, the reaction temperatures are generallyat least 30° C. and will frequently not exceed a value of 250° C. Thehydrogenation process is preferably carried out at temperatures in therange from 50 to 200° C., particularly preferably from 70 to 180° C.,and very particularly preferably in the range from 80 to 160° C. Thehydrogenation of benzene is most preferably carried out at temperaturesin the range from 75° C. to 170° C., in particular from 80° C. to 160°C.

Possible reaction gases include not only hydrogen but alsohydrogen-comprising gases which comprise no catalyst poisons such ascarbon monoxide or sulfur-comprising gases such as H₂S or COS, e.g.mixtures of hydrogen with inert gases such as nitrogen or offgases froma reformer which usually further comprise volatile hydrocarbons.Preference is given to using pure hydrogen (purity=99.9% by volume,particularly=99.95% by volume, in particular=99.99% by volume).

Owing to the high catalyst activity, comparatively small amounts ofcatalyst based on the starting material used are required. Thus, in abatch suspension process, preference is given to using less than 5 mol%, e.g. from 0.2 mol % to 2 mol %, of active metal, based on 1 mol ofstarting material. In the case of a continuous hydrogenation process,the starting material to be hydrogenated is usually passed over thecatalyst at a space velocity of from 0.05 to 3 kg/(l(catalyst)·h), inparticular from 0.15 to 2 kg/(l(catalyst)·h).

Particularly Preferred Hydrogenation Processes

The hydrogenation of aromatics comprising a regeneration is generallycarried out at a temperature of from 75° C. to 170° C., preferably from80° C. to 160° C. The pressure is generally=50 bar, preferably from 10to 45 bar, particularly preferably from 15 to 40 bar, very particularlypreferably from 18 to 38 bar.

In the present process, preference is given to hydrogenating benzene ata pressure of about 20 bar to form cyclohexane.

In a preferred embodiment of the process of the invention, the benzeneused in the hydrogenation process has a sulfur content of generally ≦2mg/kg, preferably ≦1 mg/kg, particularly preferably ≦0.5 mg/kg, veryparticularly preferably ≦0.2 mg/kg and in particular ≦0.1 mg/kg. Asulfur content of ≦0.1 mg/kg means that no sulfur is detected in thebenzene by the measurement method indicated below.

The hydrogenation can generally be carried out in the suspension orfixed-bed mode, with the fixed-bed mode being preferred. Thehydrogenation process is particularly preferably carried out withrecirculation of liquid, with the heat of hydrogenation being able to beremoved by means of a heat exchanger and utilized. The feed/recycleratio when the hydrogenation process is carried out with recirculationof liquid is generally from 1:5 to 1:40, preferably from 1:10 to 1:30.

To achieve complete conversion, an after-reaction of the hydrogenationproduct mixture can be carried out. For this purpose, the hydrogenationproduct mixture can, subsequent to the hydrogenation process, be passedin the gas phase or in the liquid phase in a single pass through adownstream reactor. In the case of a liquid-phase hydrogenation, thereaction can be operated in the downflow mode or in a flooded state. Thereactor is charged with the catalyst according to the invention or withanother catalyst known to those skilled in the art.

Regeneration Step

In hydrogenation processes in which the catalysts described above areused, deactivation is observed after a period of operation of thecatalyst. Such a deactivated ruthenium catalyst can be brought back tothe state of the original activity by flushing. The activity can berestored to >90%, preferably >95%, more preferably >98%, inparticular >99%, most preferably >99.5%, of the original value. Thedeactivation is attributed to traces or residues of water adsorbed onthe catalyst. This can surprisingly be reversed by flushing with inertgas. The regeneration method of the invention can thus also be referredto as drying of the catalyst or removal of water from this.

“Flushing” means that the catalyst is brought into contact with inertgas. Normally, the inert gas is then passed over the catalyst by meansof suitable constructional measures known to those skilled in the art.

The flushing with inert gas is carried out at a temperature of fromabout 10 to 350° C., preferably from about 50 to 250° C., particularlypreferably from about 70 to 180° C., most preferably from about 80 to130° C.

The pressures applied during flushing are from 0.5 to 5 bar, preferablyfrom 0.8 to 2 bar, in particular from 0.9 to 1.5 bar.

According to the invention, the treatment of the catalyst is preferablycarried out using an inert gas. Preferred inert gases comprise nitrogen,carbon dioxide, helium, argon, neon and mixtures thereof. Nitrogen ismost preferred.

In a particular embodiment of the invention, the inventive method ofregeneration is carried out without removal of the catalyst in the samereactor in which the hydrogenation has taken place. The flushing of thecatalyst according to the present invention is particularlyadvantageously carried out at temperatures and pressures in the reactorwhich correspond to or are similar to those in the hydrogenationreaction, resulting in only a very brief interruption of the reactionprocess.

According to the present invention, the flushing with inert gas iscarried out at a volume flow of from 20 to 200 standard l/h, preferablyat a volume flow of from 50 to 200 standard I/h per liter of catalyst.

The flushing with inert gas is preferably carried out for a time of from10 to 50 hours, particularly preferably from 10 to 20 hours. Forexample, the calculated drying time of the catalyst bed of an industrialcyclohexane production plant having an assumed moisture content of 2 or5% by weight is approximately 18 or 30 hours, respectively. The flushingaccording to the method of the invention can be carried out either in adownward direction (downflow mode) or in an upward direction (upflowmode).

The present invention further provides an integrated process for thehydrogenation of benzene in the presence of a ruthenium catalyst havinga catalyst regeneration step, which comprises the following steps:

-   (a) provision of benzene and a ruthenium catalyst;-   (b) hydrogenation of the benzene by contact with hydrogen in the    presence of the ruthenium catalyst until the catalyst has a reduced    hydrogenation activity,-   (c) regeneration of the catalyst by flushing with inert gas,-   (d) if appropriate, repetition of the steps (a) to (c).

The hydrogen used according to the invention preferably comprises nodamaging catalyst poisons such as CO. For example, reformer gases can beused. Preference is given to using pure hydrogen as hydrogenation gas.

The method of the invention is also suitable for drying catalysts whichhave absorbed water during various procedures such as maintenance orstorage.

The invention accordingly provides a method of drying and/orreactivating and/or regenerating a catalyst comprising ruthenium on asupport material, in which the catalyst is treated with an inert gas attemperatures of from 20 to 350° C. After this treatment, the catalysthas a higher catalytic activity than before.

The invention is illustrated by the following examples.

Example of the Production of the Ruthenium Catalyst

A mesoporous/macroporous aluminum oxide support in the form of 3-5 mmsphere having a total volume of 0.44 cm³/g, with 0.09 cm³/g (20% of thetotal pore volume) being formed by pores having a diameter in the rangefrom 50 nm to 10 000 nm and 0.35 cm³/g (80% of the total pore volume)being formed by pores having a diameter in the range from 2 nm to 50 nm,a mean pore diameter in the region of 11 nm and a surface area of 286m²/g was impregnated with an aqueous ruthenium(III) nitrate solution.The volume of solution taken up during impregnation correspondedapproximately to the pore volume of the support used. The supportimpregnated with the ruthenium(III) nitrate solution was subsequentlydried at 120° C. and activated (reduced) in a stream of hydrogen at 200°C. The catalyst produced in this way comprised 0.5% by weight ofruthenium, based on the weight of the catalyst. The ruthenium surfacearea was 0.72 m²/g, and the ratio of ruthenium surface area to supportsurface area was 0.0027.

EXAMPLE 1 Sorption Studies

The affinity of the catalyst for water was determined by means ofmeasurements of the sorption of water vapor on the catalyst produced asdescribed above (0.5% Ru/γ-Al₂O₃).

It was found that the catalyst sorbs an amount of water of 5% even atrelatively low vapor pressures of 30%. If only traces of water arepresent in the reactor or in the starting materials, this water can besorbed on the catalyst.

EXAMPLE 2 Operating Life Experiment in the Hydrogenation of Benzene

In a plant for the preparation of cyclohexane using a ruthenium/aluminumoxide catalyst comprising 0.5% of Ru on a γ-Al₂O₃ support, a steadydecrease in the catalyst activity and an increasing benzene content inthe product stream are observed. Further monitoring of the reactionduring a catalyst operating life test shows that the residual benzenecontent downstream of the main reactor in the hydrogenation of benzeneincreases from a few hundred ppm to some thousands of ppm over a periodof operation of about 3400 hours. A calculation indicates thatintroduction of 16 620 kg/h of benzene having a water content of from 30to 50 ppm introduces 0.8 kg of water per hour into the plant. Inaddition to this, there are a further 3.5 kg/h of water originating fromthe hydrogen gas.

When the plant was shut down after 3394 hours of operation, the plantran with a residual benzene content of 0.2% at a WHSV of 0.6g_(benzene)/ml_(cat)·h. During shutdown, the plant was flushed withpressurized nitrogen at a temperature of 70-100° C. and thendepressurized. After start-up, the plant gave a residual benzene contentof from 0.01% to 0.04% at a WHSV of 0.6 g_(benzene)/ml_(cat)·h.

This observed effect of drying of the catalyst was verified again after7288 hours of operation. At a WHSV of 0.9 g_(benzene)/ml_(cat)·h, theresidual benzene content at the end of the plant was 0.2% and even roseto 0.56%. After shutdown of the plant, the catalyst was dried by meansof 100 standard l/h of nitrogen at 110° C. for a period of 34 hours.After start-up of the plant at a WHSV of 0.6 g_(benzene)/ml_(cat)·h, theresidual benzene content was from 0.03% to 0.07%, which can beattributed to a significant increase in the catalyst activity as aresult of drying.

In both cases, drying of the catalyst led to a significantly highercatalyst activity which is close to or equal to the original catalystactivity.

EXAMPLE 3 Examination of the Influence of Water on the Hydrogenation ofBenzene

To simulate the influence of water on the hydrogenation of benzene usinga ruthenium catalyst, series of autoclave experiments before and aftersaturation of the catalyst with water and after drying of the catalystwere carried out. A 5% strength solution of benzene in cyclohexanetogether with the ruthenium catalyst was placed in the pressure vessel,the mixture was heated to the reaction temperature of 100° C. and thecourse of the reaction at a hydrogen pressure of 32 bar was followed byregular sampling. The samples were subsequently analyzed by gaschromatography.

23 hydrogenation experiments were carried out, and the catalyst wassubsequently placed in water. 13 further hydrogenation experiments werethen carried out. The catalyst displayed a significantly lower butvirtually constant activity. After drying of the catalyst in a stream ofnitrogen at 100° C. in a reaction tube, 5 further experiments werecarried out; the catalyst displayed a hydrogenation activity similar tothat before saturation with water.

The experiments demonstrate that the activity of the ruthenium/aluminumoxide catalyst used decreases significantly after contact with water,but the catalyst can be reactivated again by drying in a stream ofnitrogen and the initial activity can be virtually fully restored.

The invention claimed is:
 1. A method for hydrogenating benzene tocyclohexane consisting of the following steps: a) Providing benzene anda ruthenium catalyst, wherein the ruthenium catalyst is selected fromamong the following groups; i) A catalyst comprising, as active metal,ruthenium alone or ruthenium together with at least one metal oftransition group IB, VIIB or VIII of the periodic table of the elementsin an amount of from 0.01 to 30% by weight, based on the total weight ofthe catalyst, applied to a support, wherein from 10 to 50% of the porevolume of the support is formed by macropores having a pore diameter inthe range from 50 nm to 10,000 nm and from 50 to 90% of the pore volumeof the support being formed by mesopores having a pore diameter in therange from 2 to 50 nm, with the sum of the pore volumes being 100%; andii) A coated catalyst comprising, as active metal, ruthenium alone orruthenium together with at least one further metal of transition groupIB, VIIB or VIII of the Periodic Table of the Elements applied to asupport comprising silicon dioxide as support material, wherein theamount of active metal is <1% by weight, based on the total weight ofthe catalyst, and at least 60% by weight of the active metal is presentin a shell of the catalyst to a penetration depth of 200 μm, determinedby means of SEM-EPMA (EDXS); b) Hydrogenating the benzene by contactwith hydrogen in the presence of the ruthenium catalyst until thecatalyst has a reduced hydrogenation activity, c) Regenerating thecatalyst consisting of flushing with inert gas until an activity of >90%of the original activity has been attained, and d) Optionally, repeatingsteps a) to c); wherein step c) is carried out without removal of thecatalyst, and wherein the pressure applied during flushing is from 0.5to 5 bar.
 2. The method according to claim 1, wherein the flushing withinert gas is carried out at a temperature of from 10 to 350° C.
 3. Themethod according to claim 2, wherein the inert gas is selected fromamong nitrogen, carbon dioxide, helium, argon, neon and mixturesthereof.
 4. The method according to claim 2, wherein the flushing withinert gas is carried out at a volume flow of from 20 to 200 standard 1/hper liter of catalyst.
 5. The method according to claim 2, whereinflushing with inert gas is carried out for a time of from 10 to 50hours.
 6. The method according to claim 1, wherein the inert gas isselected from among nitrogen, carbon dioxide, helium, argon, neon andmixtures thereof.
 7. The method according to claim 6, wherein theflushing with inert gas is carried out at a volume flow of from 20 to200 standard 1/h per liter of catalyst.
 8. The method according to claim6, wherein flushing with inert gas is carried out for a time of from 10to 50 hours.
 9. The method according to claim 1, wherein the flushingwith inert gas is carried out at a volume flow of from 20 to 200standard 1/h per liter of catalyst.
 10. The method according to claim 1,wherein flushing with inert gas is carried out for a time of from 10 to50 hours.
 11. The method according to claim 1, wherein the catalyst is acatalyst comprising, as active metal, ruthenium alone or rutheniumtogether with at least one metal of transition group IB, VIIB or VIII ofthe Periodic Table of the elements in an amount of from 0.01 to 30% byweight, based on the total weight of the catalyst, applied to a support,wherein from 10 to 50% of the pore volume of the support is formed bymacropores having a pore diameter in the range from 50 nm to 10 000 nmand from 50 to 90% of the pore volume of the support being formed bymesopores having a pore diameter in the range from 2 to 50 nm, with thesum of the pore volumes being 100% and the at least one metal oftransition group IB, VIIB or VIII of the Periodic Table of the elementsis platinum, copper, rhenium, cobalt, nickel or a mixture of two or morethereof.
 12. The method according to claim 1, wherein the catalyst is acatalyst comprising, as active metal, ruthenium alone or rutheniumtogether with at least one metal of transition group IB, VIIB or VIII ofthe Periodic Table of the elements in an amount of from 0.01 to 30% byweight, based on the total weight of the catalyst, applied to a support,wherein from 10 to 50% of the pore volume of the support is formed bymacropores having a pore diameter in the range from 50 nm to 10 000 nmand from 50 to 90% of the pore volume of the support being formed bymesopores having a pore diameter in the range from 2 to 50 nm, with thesum of the pore volumes being 100% and the support is activated carbon,silicon carbide, aluminum oxide, titanium oxide, zirconium oxide,magnesium oxide, zinc oxide or a mixture of two or more thereof.
 13. Themethod according to claim 1, wherein the catalyst is a coated catalystcomprising, as active metal, ruthenium alone or ruthenium together withat least one further metal of transition group IB, VIIB or VIII of theperiodic table of the elements applied to a support comprising silicondioxide as support material, wherein the amount of active metal is <1%by weight, based on the total weight of the catalyst, and at least 60%by weight of the active metal is present in the shell of the catalyst toa penetration depth of 200 μm, determined by means of SEM-EPMA (EDXS)and the at least one metal of transition group IB, VII B or VIII of theperiodic table of the elements is platinum, rhodium, palladium, iridium,cobalt, nickel or a mixture of two or more thereof.